2.4 Exposure
The primary exposure of interest was baloxavir prescription. We selected oral and inhaled neuraminidase inhibitors (NAIs) as the active comparator, namely oseltamivir, zanamivir and laninamivir. Intravenous NAI treatment was not considered as a comparator because recipient for this form might have the different characteristic (eg, severity) from those receiving baloxavir and other forms of NAIs. Within the same disease course, a few patients (<0.1%) received a different type of antiviral drug or more than one type. We assigned these patients to the antiviral group according to the first prescribed class (since the second drug was unlikely to be prescribed within 48 hours of symptom onset). All prescribed drugs, including antivirals, were identified by drug code and brand name (when necessary) from the database. All anti-influenza drugs investigated are covered by insurance for the treatment of influenza, but not covered for prevention.